When I set the "(Guest)" user account and disable the "See Channels" option in the "Display" tab, then as desired I find that guests who login without a password cannot see any channels other than those in the root-level lobby.

Regretably however, I also find that:

Guests ONLY see the users who are also in the lobby when they arrive.

Guest do not see (or subsequently hear) any users present in private rooms, whom then do see they arrive and then switch to that room to speak with them.

Although guests do see any registered users who log into the vent lobby after them, they do not see those user ids disappear from vent when a user subsequently switches to another channel.